Apparatus for detecting strain in glass of a partially diffusing character



. s. M. GRAY 2,073,690 APPARATUS FOR DETECTING STRAIN IN GLASS March 16, 1937.

2 Sheets-Sheet 1 OF A PARTIALLY DIFFUSING CHARACTER Filed April 22, 193

T 63 Q JEZIENTOR- av I Y a ATTORN S. M. GRAY 2,073,690 APPARATUS FOR DETECTING STRAIN IN GLASS March 16, 1 937.

7 OF A PARTIALLY DIFFUSING CHARACTER 2 Sheets-Sheet 2 Filed April 22, 1932 INVENTOR- Z. 61.47

. ATTORNEY- ture.

Patented Mar. 16, 1937 UNITED STATES APPARATUS FOR DETECTING STRAIN IN GLASS OF A PARTIALLY DIFFUSING CHARACTER Samuel M. Gray, Yonkers, N. Y., assignor to Electrical Testing Laboratories, New York, N. Y., a corporation of New York Application April 22, 1932, Serial No. 606,851

4 Claims. (Cl. 88-14) The present invention relatesto apparatus, commonly referred to as polariscopes, for detecting strains in glass. K The invention is particularly concerned with the detection of strain in frosted glass (clear glass having an irregular surface produced by acid etching or sand blasting) and opalescent or lightly flashed opal glass (clear glass having on one of its surfaces a flashing of opal glass). The invention may be applied to any Y clear glass having a surface or thin film of a light-diffusing character. Strains in glass are largely due to improper an healing or too rapid cooling, the most typical of all annealing strains being those arising from the fact that the outside surface of the glass cools beforethe inside surface. -In the manufacture of electric lamps, strains in theglass bulb are liable to occur as a result of improper blowing or cooling. However, strains of a more serious character and more frequent occurrence have their origin subsequently to the manufacture of the lamp proper, when annealing a comparatively small portion of the bulb for a reason which will be presently understood. It is desirable to mark or characterize the finished lamp with reference to its' make or the manufacturerstrade-mark and with reference to its rated voltage and wattage. The usual procedure for-such marking of the lamp consists in stamping the marks in ink on the bulb, thereafter subjecting the marked portion to a fixing process consisting in the application of a flame to said marked portion, and finally slowly cooling the bulbto room tempera- If this heating and cooling is not'carried out with care, comparatively severe strains may be produced in theportion of the glass so treated.

Besides the disadvantage that lamp bulbs are rendered more susceptible to breakage if strains are present in the glass, necessitating increased precaution in handling the lamps, strains becomeof a dangerous nature when present in thebu'lbs of gas filled lamps. As is well known, upon breakdown or rupture under service of the filamerit of a gas-filled lamp, an arc may beset" up through the gas within the envelope, which are is generally heavy enough to cause the melt? ing of those portions of the filament which are bridged over by the arc. As the resistance of the circuit decreases, the strength of the arc steadily increases, with'lthe result that the complete extent of the filament and its supporting electrodes leading thereto may be caused to melt.

Should this hot fused metal strike an envelope in which a strain is present, the impact as well as the sudden envelope may cause the latter to-break with an attendant explosion of great violence. It is thus desirable that electric lamps be tested for glass strain prior to their being placed on the market.

The glass industry has generally adopted two polarization methods for detecting strain in glass, the one applying to the testing of clear or transparent glasses and the other applying to the testing of either transparent glasses or glasses which are of a semi-opaque or light-diffusing character. The first of these methods offers no special difliculties, and, briefly stated, consists in viewing the specimen undergoing test between two polarizers which have their planes of polarincrease in temperature of the ization to light from a light source at right angles to one another and thus normally substantially prevent any light from being transmitted to the eye. Any birefringence as caused by strain in the specimen causes a depolarizing action on the light passing through the strained portion of the specimen, which lights up the field locally, giving both the location and an estimate of the magnitude of the strain. It is to be particularly noted that a diffuser such as a sheet of ground glass or opal glass is required between the first polarizer and the light source, in order that the field in which the specimen is placed may be illuminated uniformly.

Heretofore, this visual examination of the specimen has been considered impossible when working with translucent or light-diffusing glasses, which redirect the visible rays of the spectrum. Thus the method generally adopted for testing such glasses, which is the second of the methods above referred to, utilizes'the heat eflect of the.

short infrared rays of the spectrum, which rays are capable of being transmitted without diffusion through either transparent or opaque glasses. The specimen is again interposed between two polarizers, but this radiation method requires additional apparatus of an expensive and delicate nature, such as a'thermopile on which the rays which may pass through the second polarizer are concentrated and agalvanometer connected to the thermoplle for indicating Still another object of the invention is to provide an apparatus of simple construction for visually detecting strain in either transparent bulbs of a partially diffusing character.

The invention will be clearly understood from the following description taken in connection with the accompanying drawings, in which- Figs. 1 and 2 are schematic illustrations of embodiments of the invention; Fig. 3.is a verticalcross-sectional view through the central plane of an apparatus shown adapted for the testing of electric lamp bulbs of a partially diffusing character; Fig. 4 is a similar view of the same apparatus shown adapted for the testing of transparent electric lamp bulbs; Fig. 5 is a fragmentary cross-sectional view of the apparatus taken on the plane indicated by the line 5 5 of Fig. 3; Fig. 6 is a fragmentary front view of the apparatus, showing the rotatable disc having the analyzer mounted therein; Figs. '7 and 8 are reproductions of actual photographs of bulbs of a partially diffusing character, taken under test.

I have made the important discovery that glass having a surface or film of a light-diffusing character may be visually examined for strain between two polarizers having their respective planes of polarization to light at right angles to one another, by utilizing a light source of extremely high intensity and causing the rays from the latter to be directly incident without diffusion, to the first polarizer. Thus, I omit the diffuser which has heretofore been considered essential between the source of light and the first polarizer, so that, by this omission, I am able to cause a powerful beam of non-diffused plane-polarized light to issue from the first polarizer, With the specimen of the glass mentioned positioned for test in the path of such beam, strain therein may, as I have found, be properly visually detected, notwithstanding the diffusion produced by the light-diffusing surface or light-diffusing film portion of the specimen. On the other hand, this novel method of visually detecting strain in glass of the type referred to is made possible by the very nature of said surface or film portion, as said surface or film portion acts to even up the illumination of the visual field in which the strain appears. That is, a field or background of uniform dimness is created in which any bright areas caused by strain in the specimen are made apparent to the eye by contrast therewith. The rays from the first polarizer pass through the specimen to the second polarizer without diffusion other than that caused by the light-diifusingsurface or light diffusing film portion, but if 'the light source is of sufllciently high intensity, a substantial quantity of rays, as I have found, pass substantially undiffused through the light-diffusing surface or "light-diffusing film portion, because of which visual detection of strain in the specimen is made possible. For convenience in description hereinafter, the first and second polarizer will be simply referred to as the polarizer and analyzer, respectively, these being the terms usually employed in the art for the two polarizers.

In Fig. 1 I have illustrated in simplified manner a means for carrying out the above method of strain detection. A powerful source of light I is placed at the principal focus of a converging lens 2 ,to produce a beam of parallel rays 4. In the embodiment illustrated a reflector, such as a 75 polished plate of black Carrara glass, 8 us d as the polarizer, which plate 5 is placed in the direct path of the powerful beam of light I, the reflected rays 6 being plane-polarized with reference to their planes of reflection, or, since those planes are parallel in the embodiment illustrated, to the central plane of refiection (coinciding with the plane of the drawings). That is, the vibrations of each ray 6 are in a plane which may be conveniently assumed to be perpendicular to the plane in which it is reflected. Preferably, the angle of incidence a of the beam 4 to the plate 5 is made equal to the polarizing angle of the substance of which the plate 5 is made. The polarizing angle of a substance is the angle of incidence causing maximum polarization andis equal to the angle the tangent of which equals the refractive index of the substance. For black Carrara glass, this polarizing angle is 56 degrees and 40 minutes. The analyzer, a Nicol prism I in the embodiment illustrated, is placed in alinement with the planepolarized rays 6. A Nicol prism is constructed from an elongated rhombohedron of Iceland spar cut into two parts along a certain plane, which parts are cemented together by Canada balsam. Iceland spar is a double-refracting crystal, i. e., it gives rise under certain conditions to two refracted rays when a single light ray is incident thereto. One of these two refracted rays is called the ordinary ray and the other the extraordinary ray, and the two rays are plane-polarized in planes at right angles to one another respectively. Referring to Fig: l, first assuming that both ordinary rays and extraordinary rays are produced in the Nicol prism 1 by the beam of light incident to the end face a of said prism upon entering said face, said prism acts to intercept all of the ordinary rays by total reflection from the Canada balsam, but to permit the extraordinary rays E to pass and reach the eye, for the sake of clarity only one ordinary ray and one extraordinary ray, corresponding to the center incident ray 8, being illustrated. It should be noted that the extraordinary rays E are plane-polarized at right angles to the principal planes of the prism taken with reference to its end faces a and b. A principal plane of a crystal with reference to one of its faces may be defined as a plane which is drawn at right angles to that face and in which a ray of unpolarized light can proceed without double refraction or in which polarized light can travel without having its polarization disturbed. Thus, if the prism 'I be turned about the center incident ray 8 as axis until its central principal plane with reference to its end faces a and b coincides with the central plane of reflection of the polarized rays 6 (the plane of the drawings), the planes of polarization with reference to which the analyzer I would polarize light assumed traveling therethrough are then perpendicular to the plane of the drawings or at rightangles to the planes with reference to which light from the source I is polarized by the reflector 5. For this position of the analyzer (assumed to be the case in Fig. l) it is set to reject the larger portion of the light that the polarizer sends forward. That is, no extraordinary rays E are theoretically capable of production and the ordinary rays 0 are lost by total reflection, so that minimum illumination of the visual field is produced.

Now. as I have discovered, it is possible to de tect strain in a specimen such as the plate 9 having a surface or film ll of a light-diffusing character, by interposing it between the polarizer 5 and face or light-diffusing film portion of the specithe analyzer 1 in the path of the powerful beam of plane-polarized rays 5. Strained glass is birefringent, -i. e., it acts as a double-retracting. crystal under the influence of light. Thus, a 5 strain in the specimen causes a change in the character of those rays 6 which travel through said strain. This change is generally believed to be a transformation of the plane-polarized light leaving the polarizer into elliptically-polarized 10 light. Whatever may be the nature of the change,

rays E are transmitted to the eye and the field is illuminated locally, the illuminated portions corresponding to strained portions of the glass. It is to be particularly noted that successful ex- 5 amination of the specimen 9 is made possible by omitting the usual diffuser between the source I and the polarizer 5 and by illuminating the polarizer 5 by properly directed rays from the powerful light I. Any reflection or refraction suffered 0 by polarized light changes its plane of polarization. Thus, should the polarizer 5 be illuminated by diffused light, the specimen 9 would likewise be illuminated by diffused light and, because of the diffusing power of the surface or film II, much of the light reflected by the polarizer 5 in the direction of the analyzer 1 would be redirected elsewhere and much of the light reflected by the polarizer 5 in other directions would be redirected toward the analyzer 1 after suffering considerable deviation and change in plane of polarization. the net result being depolarization cf the light incident to the analyzer 1.- On the other hand, by so directing the rays from the source I as to cause them upon transmission from the polarizer 5 to normally all proceed in a general direction toward the analyzer 1, the light incident to the analyzer 1 suffers a minimum of reflection or refraction when passing through the surface or film I I. As

1 have found, if the source I is of sufllciently.

high intensity, substantially undepolarized light is forced through the surface or film II in quantity sufficient to have the proper detecting action in the visual field on any strain in the specimen. It is to be particularly noted. however, that this strain detection is actually permitted by the surface or film II, because of the diffusing power of which all rays reaching the analyzer and composing the visual fleld'are made uniform in intensity, acts to even up both location and magnitude of strain (by its degree of illumination) being therefore made distinctly observable in the visual field. I have found it possible to detect strain in the specimen whether positioned, as in Fig. 1, with the plane-polarized rays 6 incident to its light-diffusingsurface or light-diffusing film portion II, or reversed from its position shown in said figure, with said rays 5 incident to its transparent portion or to the glass 0 proper. 7

With the analyzer 1 positioned in the path of the reflected rays as in Fig. 1, light is to a certain extent transmitted through the analyzer 1 to the eye under any conditions, even with the specimen 9 removed from the apparatus. The greater the intensity of the source I, the greater is the amount thus transmitted. Moreover, the amount transmitted is appreciably, increased when a tint-producing element (to be hereinafter described) is interposed between the specimen and the analyzer. This transmitted light, although not effecting detection or visibility of strain because of the uniform illumination of the field which results from the light-diffusing surtherefore conducted directly to the eye.

men, becomesparticularly objectionable, in view of its intensity and the resulting harm to the eye, when the specimen'is removed from the line of vision, no light-diffusing means being then present anywhere along the path of travel of the light from the powerful source I. Now, I have found it possible to view the specimen under study at an angle with the direct path of the. rays sentforward by the polarizer, so that none of these rays are incident to the analyzer and none A clear determination of any strain is nevertheless made possible by reason of the intense illumination of the specimen. This indirect method of viewing the specimen is illustrated in simplified manner in Fig. 2, and is the method I prefer when testing such specimens as electric lamp bulbs of frosted glass or the like. The apparatus includes the same elements as in Fig. 1, but here the analyzer 1 is shown as having been rotated degrees about an axis I2, making an angle with the center'reflected ray 9, to bring the line of vision out of the direct path of the reflected rays 5. The specimen Il, shown as an electric lamp bulb having an inner surface or film I5 of a light-diffusing character, may thus be conveniently held in the hand and turned into various positions in the path of the reflected rays 6 to permit its entire extent to be examined from one portion to another, without danger to the eye from the direct rays which may travel past said specimen unobstructed or unintercepted by said surface or film I6. By its 180 degree rotation, the analyzer 1 again has its central principal plane with reference to its .end faces a and b coinciding with the central plane of reflection of the rays 6. As in Fig. 1, therefore, the polarizer 5 and the analyzer 1 have their respective planes of polarization to light at right angles to one another, so that minimum light is normally transmitted to the eye.

The intensity ofthe light I permits rays 6 to travel through both rear and front walls of the bulb I4, so that the entire extent of said bulb is brilliantly illuminated. As has already been explained, light rays 6 undergo a certain change when passing through a strained portion of a specimen. I have discovered that this effect is transmitted to the indirect rays, such as I5, travelling from the specimen I4 to the analyzer 1 in Fig. 2. That is, a change in the character of the light is also effected on these indirect rays, when issuing from a strained portion of the specimen I4, so that light. E is correspondingly locally transmitted through the analyzer, rendering the strain visible to the eye. It should be. noted that there are two port ons of the bulb located on the line of vision. so that there are actually two portions under observation at any time. Thus, the bulb I4 should be manipulated into various positions to make sure that a single strain is visualized at one time, and not two superimposed strains which might appear to the eye as a single strain. In general, however, a single strain appears, which strain, for reasons already advanced. is in the end portion 0 of the bulb, whereon the manufacturer's stamp is located. In Fig. 2 this end portion 0 is shown under observation, it being understood that bulb-s having an outer surface or film of a light-diffusing character may be similarly tested.

' The tint-producing element I1 shown disposed in the path of the rays to the analyzer 1 in Fig. 2, consists of a thin crystalline plate or disc of a 15 through the crystalline plate.

double-retracting crystal, such as selenite,-having its principal planes set at 45 degrees to the central plane of reflection of the plane-polarized rays 5 proceeding from the polarizer 5. This thin 5 crystal l1 may be conveniently held between two plates or discs |9 of unstrained optically plane transparent glass. It acts to effect elliptic polarization of the plane-polarized rays incident thereto, due to the fact that the ordinary and extra- 10 ordinary components produced by the splitting up of the plane-polarized rays, which components have their respective planes of-vibration at right angles to one another, are unequally retarded, i. e., their velocities are different, when passing The ellipticallypolarized rays emerging from the crystalline plate l1 give rise to two sets of rays'when entering the Nicol prism. One set, the ordinary rays 0', are extinguished by total reflection, as previously explained, and theother set, the extraordinary rays E, are transmitted to the eye. These transmitted rays are made up of components of both ordinary and extraordinary rays produced by the crystalline plate l1, these components being out of phase with each other due to the retardation of one relative to the other in transmission through the selenite plate. This phase difference varies with the wave-length of the light. Consequently, when white light is used as the source I, the relative intensities of its component colors are changed when travelling through the Nicol prism, and therefore the transmitted rays E will have a prevailing tint depending on the thickness of the plate. When using selenite, the field which is usually caused to be produced is of a purplish color. If now a specimen of strained glass be introduced, it will have the effect of changing the phase difference of the superimposed ordinary and extraordinary rays transmitted through the plate H, with the result that the strain appears v in colors contrasting with that of the field.

The apparatus illustrated in Figs. 3 and 4 may be utilized for testing either transparent bulbs or bulbs of a partially diffusing character, and I" 46 have designed this apparatus to permit 7} it to be quickly altered to suit the particular test desired. This apparatus comprises an enclosing casing 20 having a base 2|, side walls 22 (one being shown,

in Figs. 3 and .4 and the two in Fig. 5), a top plate 24, and a rear end cover 25; The latter is preferably hinged to the base 2| to provide access into the rear end portion of the casing. Two

partitions 25 and 21 are inclined to one another and to the base 2|, which partitions are each provided with an opening through'whlch light may be transmitted. Each of the side walls 22 has a front edge portion parallel to the partition 25 and to which a front plate 29 is secured. The partitions 28 and 21 and the front plate 29 divide go the casing into three compartments, the front compartment of which is opened at its bottom to permit insertion of the specimen and'its manipulation in the path of the light rays, transmitted from the light source 32 in the rear compartment and reflected from the polarizer 28 in the middle compartment.- The reflector 28 is .flxed to the base 2| in any appropriate manner, and accumulation of dust particles on its polished surface is prevented by closing the opening in the partition 28 by a transparent plate of unstrained glass 39. If black Carrara glass is used for the reflector 28, the angle of the partitions 26 and 21 with the base I is preferably made equal to 56 degrees and 40 minutes, which is the polarizing angle of that substance, as already stated. The

light-transmitting apparatus located in the rear compartment is adjustably mounted on an inclined block 3| and comprises a lamp of high intensity, such as a 400 watt projection lamp, to which current is supplied by the cable 33, and a concave reflecting mirror 34, both the lamp and the reflector being adjustable in perpendicular directions relatively to the fixed block 3|. A plurality of ventilation openings 35 may be formed through the top plate 34 of the casing.

When testing electric lamp bulbs of a partially diffusing character, I utilize an optical means, which may consist of a pair of planoconvex lenses 36 and 31 held in proper alinement in-a frame 38, for directing a powerful beam of parallel rays to the polarizer 28 (as shown in Fig. 3). When testing transparent bulbs, this system of lenses is removed and I utilize a light diffuser 39 for the purpose of providing a uniformly bright background. This difluser may consist of a sheet of flashed opal glass held at its edges in a suitable frame 40 and, when properly positioned for a test on a transparent bulb, completely covers the opening formed through the partition 21 (as shown in Fig. 4).

To facilitate this interchange of parts, the system of lenses 36 and 31 and the diffuser 39 are each made slidable in a separate plane. Thus, the frame 45 holding the light diflfuser is guided between the partition 21 and suitable strips 4| secured to the side walls 22 of the casing 20 (Fig. 5), while the frame 38 holding the system of lenses is provided with side flanges 42 (Fig. 5), which slidably cooperate with guides or strips 44 also flxed to the side walls 22 of the casing 20. A pair of chains 45 is attached at one end to the sides of the frame 40 ,and at the other end to.the

.sides of the frame 38, each of said chains 45 meshing with a sprocket wheel 46 fixed to a shaft 41 journaled in suitable bearing supports 49 depending from the top cover plate24. The shaft 41 has one end projecting through one of the side walls 22, and to this end is attached a crank 50 by 'into operative position (Fig. '3), and a counterclockwise movement of the handle 58 will raise the system of lenses and lower the light difiuser (Fig. 4). Suitable stops 5| and 52are provided for limiting the downward movements of the light diffuser and of the system of lenses, respectively.

A disc 54 (Fig. 6) is rotatably mounted on an inclinedblock 55 fixed. to the front plate 29 of the casing 20, and into an opening formed through this'disc 54 is fltted a tubular protective casing 55 for a Nicol prism 55 and athin sheet of selenite (held between glass plates 51). The axis of rotation of the disc 54 is at an angle to the path followed by polarized light'from the reflector 28, and at one side of this axis there is an opening 59 formed through the plate 29 and the block 55 in alinement with the center ray (58 in Fig. 3 and 58 in Fig. 4) of such light. This opening 59 is in, registration with the disc opening when the disc is rotated into the position shown in Fig.

4. There is another opening 60 formed through the plate 29 and the block 55 at the other side of the axis of rotation, which opening 6|! is in registration with the disc opening when the disc is rotated degrees from its position shown in Fig. 4 into the position shown in Fig. 3.

Thus, in Fig. 3, the line of vision through the alyzer 5G is out of alinement with the powerful beam of polarized light, which is caused to be established by employing the system of lenses 3G and 31, such specimens as the inside frosted bulb 62 shown being visually examined, as heretofore ex- 6 plained, by means of the indirect rays, such as 6 I transmitted therefrom to the analyzer. In Fig. 4, the line of vision through the analyzer 56 coincides with the center ray ,58 of polarized light so that, with the diffuser 39 properly interposed in the 10 path of the rays from the lamp 32, such specimens as the transparent bulb 63 shown may be visually examined, in the usual manner, by means of that part of the light sent forward by the diffuser 39 which is reflectedat the polarizing angle by the reflector 28. It is noted that in both positions of the Nicol prism 56, its central principal plane coincides withthe central plane of reflection, and thatin both positions of the selenite crystal, its central principal plane is set at 45 degrees with the central plane of reflection,

tests on either transparent specimens or specimens of a, partially diffusing character being thus possible without any change in the position of the prism or of theselenite crystal, except that produced by a rotation of the disc 54. All that is required for an interchange of test is a rotation of the crank 50 and a rotation of the disc 54. Fig. 7 is reproduction of an actual photograph of a typical so-called Maltese cross-shaped strain in the end portion of an inside frosted lamp, as revealed'by the apparatus of the invention. This photograph was taken with a panchromatic plate, with the tint-producing element or selenite crystal removed from the polariscope to obtain in the first instance a visual effect in black and White and therefore to permit the camera to receive a true record of the dark and bright portions of the specimen under test. Also, in order to visualize more clearly the effect of strain, I,

40 removed by crocus cloth the manufacturer's stamp which was located on the very end portion of the bulb where'the strain occurred as revealed by the polariscope. As shown in the figure, four bright spots, 10 and 'II, were caused to appear,

separated by a dark cross 12. The cross had the characteristic blackness of the field 14 in which the specimen was visualized, but this does not indicate that the stresses were zero in the cross. It should be observed that the stresses in glass are apt to be oriented in difierent'directions in different places, and that plane-polarized light is not afiected, when passing through a strained portion, if the planes in which the light rays are vibrating are parallel to or at right angles to the 65 stress. To obtain a maximum effect on the planepolarized light, its plane of polarization should be at 45 degrees to the stress. Thus, the stresses in the end portions of the cross 12 were no different in magnitude from thosein the bright spots 60 (I0 and H), but the stresses in one set of oppositely disposed end portions and those in the other set were respectively parallel and'at right angles to the plane-polarized light in which the specimen was examined, neither one of these end '65 portions therefore being capable of effecting a change in such light and capable of having much light transmitted therefrom through the analyzer.

' On the other hand, the plane-polarized light did undergo a change, probably elliptic polarization,

70 upon travelling through the strained portions where the bright spots (Ill and 'II) appeared, the

orientation of the stresses in such portions being such as to permit their illumination. .At the center of the cross I2, there was a considerable 76 dark area. Actually, the strain was maximum at this center portion, but as it was equally oriented in all directions, it could not be made to appear in the polariscope. The investigator thus uses precaution in the setting of a, specimen in the path of the plane-polarized rays, so as not to overlook the stresses altogether, as well as in interpreting the results obtained. With the selenite crystal inserted in the polariscope, the color effect was as follows: The field M was purple and the cross 12 was of a purplish hue. A blue color prevailed in one set (10) of diagonally disposed spots, indicating such orientation of stress as to have caused an increase in the phaseangle of the two component vibrations of the ellipticallypolarized light issuing from the selenite crystal. A red color prevailed in the other set (i I) of diagonally disposed spots, indicating such orientation of stress as to have caused a decrease in the phaseangle of the two component vibrations of the elliptically-polarized light issuing from the selenite crystal.

Fig. 8 illustrates the effect produced by photoother inside-frosted lamp under test in the apparatus of the invention. In this test, the selenite crystal was properly positioned in the path of the rays to the camera, and the photograph was taken with an orthochromatic plate (sensitive to blue). It should first be noted that this photographic effect is vastly different fromthe' colored efiect as it actually appeared to the eye. Here the stresses were so oriented as to cause a circular colored band concentric with the central port1on 80 where the manufacturer's mark is located, the colorred prevailingin the oppositely disposed segments 8| and the color blue prevailing in the other oppositely disposed segments 82. Thelrud color of the portions BI is the reason why they show dark in the reproduced photograph. Although the central portion 80 appears bright in 'the illustration, its color to the eye was of the characteristic purple of the field 83. This again does not indicate that the stresses were zero at the center. As a matter of fact the center was under the severest strain, but as the stresses were oriented in all directions, substantially no change was effected in the plane-polarized light trans- 'mitted through it.

The bright spots located at the rim of the two specimens in Figs. '7 and 8 were not strains in the glass, but were caused by reflection of light from the side portions of the bulbs.

} It is understood that the apparatus described herein is subject to various modifications without departing from the spirit of the invention.

What is claimed is:

1. An apparatus for visually detecting strain in glass having a surface or film of a light-diffusing character, comprising a source of light, optical means for directing rays from said source in the form of a non-diffused beam of light, a polarizer to which said beam is incident, and an analyzer adapted for movement into either of two operative positions corresponding to which the lines of vision respectively established are in and out of the path of the polarized light proceeding from said polarizer, said source being of sufficiently high intensity to cause a substantial quantity of substantially undifiused rays to pass through the light-diffusing surface of light-diffusing film portion of the specimen undergoing test in said path and on either of said lines of vision, said polarizer and analyzer for either of said analyzer positions having their respective planes of polarization to light at right angles to one another.

2. An apparatus for visually detecting strain in glass having a surface or film of a light-diffusing character, comprising a source of light, optical means for directing rays from said source in the form of a non-diffused beam of light, a polarizing reflector to which said beam is incident, and an analyzing prism adapted for rotation about an axis inclined to the axis of the polarized beam proceeding from said reflector into either of two operative positions corresponding to which the lines of vision respectively established are in and out of the path of said polarized beam, said source being of sufiiciently high intensity to cause a substantial quantity of substantially undiflused .rays to pass through the light-diflusing surface or vlight-diilusing film portion of the specimen undergoing test in said path and on either of said lines of vision, said prism in either of said positions having its planes of polarization to light at right angles to those of said reflector.

3. An apparatus for visually detecting strain in either transparent glass or glass having a surface or film of a light-diffusing character, comprising a source of light, a polarizer for polarizing rays from said source, light-diffusing means for use incarrying' out tests on specimens of transparent glass, optical means for use in carrying out tests on specimens of glass having a surface or film of a light-difi'using character, said light-diffusing means and optical means being each adapted for movement in and onto! operative position with relation to said source, said light-diffusing means operating to difluse the rays transmitted to said polarizer and said optical means operating to direct the rays transmitted to said polarizer in the form of a non-diffused beam of light, and an analyzer adapted for movement into either of two operative positions corresponding to which the lines of vision respectively established are in and out of the path followed bypolarized light from said polarlzer, specimens of transparent glass being positioned for test in said path and on substantially undiflused rays to pass through the specimens of glass having a surface or film of a light-diffusing character, said polarizer and analyzer for either of said analyzer positions having their respective planes of polarization to light at right angles to one another.

4. An apparatus for visually detecting strain in either transparent glass or in glass having a surface or film of a light-diilusing character, comprising a source of light, a polarizing plate for polarizing rays from said source by reflection, light-diffusing means for use in carrying out tests on specimens of transparent glass, optical means for use in carrying out tests on specimens of glass having a surface or film of a light-diffusing character, said light-diffusing means and optical -means being adapted for movement in separate planes to permit movement of either in and out of operative position with relation to said source and also adapted for simultaneous movement in opposite directionsto permit movement of one into operative position with relation to said source simultaneous with movement of the other out of operative position with relation to said source,

, said light-diffusing means operating to diffuse the rays transmitted to said plate and said optical means operating to direct the rays transmitted to said plate in the form of a non-diffused beamof light, and an analyzing prism adapted for rotation about an axis inclined to the path followed by polarized light from said plate into 'for test in said path and on the line of vision established out of said path, said source being of sufficiently high intensity to cause a substantial quantity of substantially undifiused rays to pass through the specimens of glass having a surface or film of a light-diffusing character, said prism in either of said positions having its planes of polarization to light at right angles to those of said plate.

1- SAMUEL M. GRAY.

CERTIFICATE OF' CORRECTION.

PatentNo. 2,073,690.

March 16, 1937.

SAMUEL ii; an

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Page '3,

first column, line 50, strike out the words ."acts to even up"; page 4,

second column, Line 75, for "alyzer" read analyzer; page 5, second 001-,

umn, line 3, after the word "investigator" insert must; and for "uses" read use; line 70, claim 1, for "of" read or; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed "this 11th day of May, A. D. 1937.

(Seal) Henry Van Arsdale 2. An apparatus for visually detecting strain in glass having a surface or film of a light-diffusing character, comprising a source of light, optical means for directing rays from said source in the form of a non-diffused beam of light, a polarizing reflector to which said beam is incident, and an analyzing prism adapted for rotation about an axis inclined to the axis of the polarized beam proceeding from said reflector into either of two operative positions corresponding to which the lines of vision respectively established are in and out of the path of said polarized beam, said source being of sufiiciently high intensity to cause a substantial quantity of substantially undiflused .rays to pass through the light-diflusing surface or vlight-diilusing film portion of the specimen undergoing test in said path and on either of said lines of vision, said prism in either of said positions having its planes of polarization to light at right angles to those of said reflector.

3. An apparatus for visually detecting strain in either transparent glass or glass having a surface or film of a light-diffusing character, comprising a source of light, a polarizer for polarizing rays from said source, light-diffusing means for use incarrying' out tests on specimens of transparent glass, optical means for use in carrying out tests on specimens of glass having a surface or film of a light-difi'using character, said light-diffusing means and optical means being each adapted for movement in and onto! operative position with relation to said source, said light-diffusing means operating to difluse the rays transmitted to said polarizer and said optical means operating to direct the rays transmitted to said polarizer in the form of a non-diffused beam of light, and an analyzer adapted for movement into either of two operative positions corresponding to which the lines of vision respectively established are in and out of the path followed bypolarized light from said polarlzer, specimens of transparent glass being positioned for test in said path and on substantially undiflused rays to pass through the specimens of glass having a surface or film of a light-diffusing character, said polarizer and analyzer for either of said analyzer positions having their respective planes of polarization to light at right angles to one another.

4. An apparatus for visually detecting strain in either transparent glass or in glass having a surface or film of a light-diilusing character, comprising a source of light, a polarizing plate for polarizing rays from said source by reflection, light-diffusing means for use in carrying out tests on specimens of transparent glass, optical means for use in carrying out tests on specimens of glass having a surface or film of a light-diffusing character, said light-diffusing means and optical -means being adapted for movement in separate planes to permit movement of either in and out of operative position with relation to said source and also adapted for simultaneous movement in opposite directionsto permit movement of one into operative position with relation to said source simultaneous with movement of the other out of operative position with relation to said source,

, said light-diffusing means operating to diffuse the rays transmitted to said plate and said optical means operating to direct the rays transmitted to said plate in the form of a non-diffused beamof light, and an analyzing prism adapted for rotation about an axis inclined to the path followed by polarized light from said plate into 'for test in said path and on the line of vision established out of said path, said source being of sufficiently high intensity to cause a substantial quantity of substantially undifiused rays to pass through the specimens of glass having a surface or film of a light-diffusing character, said prism in either of said positions having its planes of polarization to light at right angles to those of said plate.

1- SAMUEL M. GRAY.

CERTIFICATE OF' CORRECTION.

PatentNo. 2,073,690.

March 16, 1937.

SAMUEL ii; an

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Page '3,

first column, line 50, strike out the words ."acts to even up"; page 4,

second column, Line 75, for "alyzer" read analyzer; page 5, second 001-,

umn, line 3, after the word "investigator" insert must; and for "uses" read use; line 70, claim 1, for "of" read or; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed "this 11th day of May, A. D. 1937.

(Seal) Henry Van Arsdale 

